This invention relates to a separator for an alkaline battery and, more particularly, this invention relates to a cellulosic separator for a secondary zinc ion battery such as a silver-zinc battery.
Separators play a crucial role in alkaline batteries. They keep the positive and negative sides of the battery separate while letting certain ions go through and blocking others. The separator is a passive element that has to perform the same task unchanged for the life of the battery. Meanwhile, it must be able to withstand a strongly alkaline environment both at ambient and elevated temperatures. In addition, it must be capable of resisting oxidative attacks.
In an alkaline battery, a separator should conduct hydroxyl ions at a sufficiently rapid rate to meet the increasingly high current demands of modern electronics. Films of cellulose in the form of regenerated cellulose have been used since World War II as the separator of choice for this purpose because of its superior ability to conduct hydroxyl ions in strongly alkaline media. Its low electrical resistance of 10 milliohm-in2 has also led to its favor for use in zinc-based batteries, such as silver-zinc, zinc-nickel, and zinc manganese dioxide batteries. Additionally, it acts as a physical barrier to migration of other ions in the battery, such as that of zincate ions and silver ions in a silver-zinc battery.
Despite its advantages as a battery separator, regenerated cellulose has some intrinsic limitations. During overcharge, an alkaline battery tends to break down water and evolve hydrogen in sufficient quantities as to materially affect the internal impedance of the battery. Unless this hydrogen is removed efficiently, a parasitic feedback results in which the battery continues to be overcharged with resultant pressure buildup and venting of hydrogen or catastrophic rupture of the battery case. Regenerated cellulose, however, exhibits one of the lowest hydrogen permeability coefficients of known polymers, reported in the Polymer Handbook as 2.044xc3x9710xe2x88x9215 cm3cmxe2x88x921sxe2x88x921Paxe2x88x921.
Prior batteries incorporate in recombinant separators comprising porous melt-blown polymer fibers that incorporate surfactants or lubricants that facilitate gas transport within a battery. U.S. Pat. No. 6,054,084 describes separators for lead-acid batteries made of polytetrafluoroethylene (PTFE) fibril matrix incorporating particulate silica filler and non-evaporative lubricant as gas transport agents. Zucker in U.S. Pat. No. 5,962,161 describes a recombinant separator for lead-acid batteries that comprises melt-blown polypropylene made wettable by a surfactant agent thus enabling transport of oxygen.
In U.S. Pat. No. 4,919,865 Nelson teaches a method for making a composite membrane made from a mixture of polymethyl methacrylate and a cellulosic derivative, such as cellulose acetate. A gas stream containing hydrogen is selectively cleaned of the hydrogen by the presence of the methyl methacrylate. Polymethyl methacrylate is, however, unsuitable as a battery separator capable of handling high currents because of its high electrical resistance.
The separator provided by the present invention consists of a membrane having both high hydroxyl conductivity and high hydrogen transport. When the separator is placed in a silver-zinc battery, hydrogen buildup in the battery is diminished. The present invention relates to a recombinant separator that is able to transport hydrogen while conducting hydroxyl ions. The separator of the invention help maintain low electrical impedance and exhibit resistance against formation of zinc dendrites. A preferred battery separator according to the inventor contains a solution of cellulose having of a degree of polymerization between 200 and 1200 that is mixed with particles of a polymer having a hydrogen permeability greater than 1xc3x9710xe2x88x9213 cm3cmxe2x88x921sxe2x88x921Paxe2x88x921. The resulting mixture is then coagulated under controlled environmental conditions to produce a heterogeneous gel that when dehydrated yields a membrane useful as a recombinant battery separator.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.